In spectrum analysis, energy distribution of an electric signal is studied as a function of frequency of a spectrum analyzer, which allows graphic representation of the amplitude as a function of frequency in a portion of the spectrum. The analyzer may be used as a sensitive receiver to measure attenuation, FM deviation, and frequency, as well as to study RF pulses. A simple spectrum analyzer is based on a superheterodyne receiver and an oscilloscope. In the receiver, the input signal is mixed in the mixer with a frequency obtained from a voltage-controlled oscillator. The control voltage of the oscillator is a saw-tooth voltage, whereby the frequency of the oscillator sweeps over a certain frequency range. An intermediate frequency signal, obtained as the mixing result, is amplified and detected. The detected signal, which is amplified in a video amplifier, is directed to the vertical deflection plates of a cathode-ray tube and the saw-tooth voltage is directed to the horizontal deflection plates. As a result, an amplitude is shown on a display as a function of frequency. In older spectrum analyzers, the detector and the video amplifier were combined in the same block.
FIG. 1 shows a simplified block diagram of a prior art spectrum analyzer primarily intended for monitoring. An incoming RF signal is attenuated in an attenuator 1, whereafter it is low-pass filtered in a filter 2 before mixing it in a mixer stage 3 with a oscillator frequency f.sub.2. The oscillator frequency f.sub.2 is generated in a YIG oscillator block A, which comprises an oscillator 4 and its control circuits 5, 6. The frequency f.sub.2 of the YIG oscillator varies as a function of a ramp voltage obtained from a ramp generator 7, whereby the oscillator frequency sweeps over a desired frequency band. The horizontal deflection voltage of a cathode-ray tube CRT changes in response to a change in the voltage of the ramp generator 7. From the mixing results obtained in the mixer 3, a desired frequency is filtered in an adjustable band-pass filter 8, whereafter the signal frequency is further lowered in two successive mixer stages 9, 10 using fixed oscillator frequencies. The resolution band may thus be made narrower. After amplification carried out in a chain of switchable intermediate frequency amplifiers 11, band-pass filtering performed in a filter 12 with an adjustable pass band, and amplification carried out in an intermediate frequency amplifier 13, the signal is applied to a block 14 for performing a logarithmic conversion. In this block, detection is also performed. Combined component packages that carry out detection and logarithmic amplification are commercially available. Block 14 provides as an output the amplitude variation of the detected signal as a function of frequency and in accordance with the logarithmic scale. The vertical deflection voltage of the cathode-ray tube varies in response to the output of block 14, whereby the amplitude of the input signal is drawn on the tube as a function of frequency in the decibel scale.
In prior art spectrum analyzers, a video amplifier is employed. The name originates from a detector in which a logarithmic amplifier, that is, a chain of saturated amplifiers with a limiter output is used after or before envelope detection. The precision of a video amplifier of this kind is high, but its cost is high. An IF amplifier circuit with an RSSI (Received Signal Strength Indicator) circuit at the output may also be used. The advantage of amplifiers applying this solution is a low cost, but the drawback is low precision. The dynamic range that may be achieved with prior art analyzers is some 60 dB, which limits the uses of the analyzer. Furthermore, video amplifier circuits may be very costly depending on the qualities, and the variation between analyzers caused during the production is rather great, and adjustments of different kinds must thus be carried out for reducing the variation. The third drawback is caused by the fact that costly oscillator solutions, such as a YIG oscillator, must be used, since the requirements for the precision of an adjustable oscillator are high.
The object of this invention is to achieve a receiver for spectrum analysis, suited for analyzing a radio frequency signal, and to avoid the drawbacks of the prior art receivers. The object is specifically to measure the signal level as a function of frequency in a simple and reliable manner, without using a logarithmic amplification chain, and achieve a wide dynamic range, up to 100 dB.
The object is achieved with a spectrum analyzer comprising at least one mixer, a detector, an attenuator, a decoder, and a controller and recorder.